Measuring a 4-20ma Input With a Voltage Input Device

we will assume that a 0-10Vdc input will be used to measure 4-20ma signal.

Ohms law states: R=V/I where V is the Voltage, I is the current and R is the resistance

R=10V/.020A = 500 Ohms

When 20ma flows through a 500 Ohm resistor, it will drop 10 volts

When 4ma flows through a 500 Ohm resistor, it will drop 2 volts

Therefore, 4-20ma signal through a 500 ohm resister will drop 2 to 10 volts

PLC
if you’re using instrument with 4-20mA and connect it to:

1. range 0-20mA – your value of the instrument will be from that 5529 to 27648, value from 0 to cca 5500 will be not showed, but it depends on instrument.

2. range 4-20mA – your value of instrument will be from 0┬áto 27648.

References
https://www.omega.co.uk/techref/das/4-20ma.html
https://support.industry.siemens.com/tf/WW/en/posts/analog-input-evaluation/146749?page=0&pageSize=10
https://www.omega.com/en-us/resources/data-acquisition-measuring-420-input-with-voltage-device

How can the CPU analog inputs of the SIMATIC S7-1200 also measure currents of 0-20mA

s7-1200_cpu_analoginputs_current_01

Fig. 01: Connection of a sensor to a voltage source as a 2-conductor connection

s7-1200_cpu_analoginputs_current_02

Fig. 02: Connection of a sensor to a voltage source as a 3-conductor connection

s7-1200_cpu_analoginputs_current_03

Fig. 03: Connection of a sensor to a voltage source as a 4-conductor connection

s7-1200_cpu_analoginputs_current_04

Fig. 04: Connection of a measuring transducer with voltage output to a current source as a 4-conductor connection

References :
https://support.industry.siemens.com/cs/document/67396370/how-can-the-cpu-analog-inputs-of-the-simatic-s7-1200-also-measure-currents-of-0-20ma?dti=0&lc=en-WW